An electrochemical accumulator has a nominal voltage of the order of a few volts, and more precisely 3.3 V for Li-ion batteries based on iron phosphate and 4.2 V for a Li-ion technology based on cobalt oxide. If this voltage is too low with respect to the requirements of the system to be powered, several accumulators are placed in series. It is also possible to dispose in parallel with each accumulator associated in series, one or more accumulators in parallel so as to increase the available capacity and to provide greater current and power. The accumulators associated in parallel thus form a stage. A stage consists of a minimum of one accumulator. The stages are arranged in series so as to attain the desired voltage level. The association of the accumulators is called an accumulator battery.
The charging or discharging of an accumulator is manifested respectively by a growth or decay of the voltage across its terminals. An accumulator is considered charged or discharged when it has attained a voltage level defined by the electrochemical process. In a circuit using several accumulator stages, the current flowing through the stages is the same. The level of charge or of discharge of the stages therefore depends on the intrinsic characteristics of the accumulators, namely the intrinsic capacitance and the series and parallel parasitic internal resistances, of the electrolyte or of contact between the electrodes and the electrolyte. Voltage differences between the stages are therefore possible on account of the disparities of manufacture and of aging.
For a Li-ion technology accumulator, too high or too low a voltage, termed the threshold voltage, may damage or destroy the accumulator. For example, overcharging a Li-ion accumulator based on cobalt oxide may cause thermal runaway thereof and start a fire. For a Li-ion accumulator based on iron phosphate, overcharging is manifested by decomposition of the electrolyte which decreases its lifetime or may impair the accumulator. Too great a discharge which leads to a voltage of less than 2 V, for example, mainly causes oxidation of the negative electrode current collector when the latter is made of copper and therefore impairment of the accumulator. Consequently, monitoring of the voltages across the terminals of each accumulator stage is compulsory during charging and discharging for the sake of safety and reliability. A so-called monitoring device in parallel with each stage makes it possible to ensure this function.
The function of the monitoring device is to follow the state of charge and of discharge of each accumulator stage and to transmit the information to the drive circuit no as to stop the charging or discharging of the battery when a stage has attained its threshold voltage. However, on a battery with several accumulator stages disposed in series, if charging is stopped when the most charged stage attains its threshold voltage, the other stages may not be fully charged. Conversely, if discharging is stopped when the most discharged stage attains its threshold voltage, the other stages may not be fully discharged. The charge of each accumulator stage is therefore not utilized in an optimal manner, this representing a major problem in applications of transport and onboard types having strong autonomy constraints. To alleviate this problem, the monitoring device is generally associated with an equalization device.
The function of the equalization device is to optimize the charge of the battery and therefore its autonomy by bringing the accumulator stages arranged in series to an identical state of charge and/or discharge. There exist two categories of equalization devices, so-called energy dissipation equalization devices, or so-called energy transfer equalization devices.
With energy dissipation equalization devices, the voltage across the terminals of the stages is rendered uniform by bypassing the charge current of one or more stages that have attained the threshold voltage and by dissipating the energy in a resistor. As a variant, the voltage across the terminals of the stages is rendered uniform by discharging one or more stages that have attained the threshold voltage. However, such energy dissipation equalization devices exhibit the major drawback of consuming more energy than required to charge the battery. Indeed, this circuit makes it necessary to discharge several accumulators or to divert the charge current of several accumulators so that the last accumulator or accumulators, which are slightly less charged, terminate their charging. The energy dissipated may therefore be much greater than the energy of the charge or charges that has or have to be terminated. Moreover, they dissipate the excess energy as heat, this not being compatible with the constraints of integration within applications of transport and onboard types, and the fact that the lifetime of the accumulators diminishes greatly when the temperature rises.
Energy transfer equalization devices exchange energy between the accumulator battery or an auxiliary energy network and the accumulator stages.
For example, U.S. Pat. No. 5,659,237 discloses a device allowing the transfer of energy from the auxiliary network to stages through a “flyback” structure with several outputs and using a coupled inductor as storage element. The latter is a specific component because it is dedicated to this application. The cost of such a component is prohibitive with respect to the function to be fulfilled.
Moreover, patent CN1905259 discloses a device allowing the transfer of energy from the stages to the battery and which, for its part, uses one inductor per accumulator as storage element. However, this device does not opt for energy transfer that is optimized for the equalization of the batteries in applications of transport and onboard types. Indeed, the end of charging of a battery is determined by the last stage which attains the threshold voltage. To terminate the charging of a battery, the energy is tapped off from one or more stage(s) and it is returned to all the stages. When one or more accumulator stage(s) is or are slightly less charged, the energy is therefore not transferred by priority to the stage(s) which needs or need it but also to the stage(s) from which the energy is tapped off. Equalization therefore requires that energy be tapped off from all the stages at the end of charging so as to avoid charging them to too high a voltage. The equalization is therefore done with high losses on account of the large number of converters in operation. Moreover, the accumulators already at the end of charging are traversed by non-useful alternating or direct components of current.